Slaves were medieval Mediterranean traders’ most valuable commodity.

Evidence

Still, according to Iris Origo, the most precious cargo of the Mediterranean tradesmen was slaves: … European and Levantine traders sold Grecian wines and Ligurian figs, and the linen and woolen stuffs of Champagne and Lombardy, and purchased precious silks from China, carpets from Bokhara and Samarkand, furs from the Ural Mountains, and Indian spices, as well as the produce of the rich black fields and forests of the Crimea. But the most flourishing trade of all was that in slaves—for Caffa was the chief slave-market of the Levant. — Cedric J. Robinson, Black Marxism_ The Making of the Black…, 156